The health benefits of PA are widely accepted, aiding in reducing the risk of cardiovascular disease, diabetes, stroke, obesity, and some cancers. One-fourth of adults are inactive with higher rates of inactivity among racial and ethnic minorities. Individuals who live in non-supportive environments are less likely to engage in PA. This may be particularly important for understanding PA in racial and ethnic minorities, who (a) are less likely than whites to meet PA recommendations and (b) disproportionately live in communities lacking PA resources. Thus, a greater understanding of environmental factors in the natural environment (e.g., community-based settings) and how they influence PA psychosocial processes (e.g., stress) and stages (e.g., adoption and maintenance) among racial and ethnic minorities are needed to reduce disparities in PA. Therefore, the primary aim of this study is to use survey, qualitative, and state-of-the-science ecological momentary assessment (EMA) methods to longitudinally examine the influence of selected psychosocial, social contextual, and objective and perceived physical environmental factors on self-initiated lifestyle moderate-intensity PA in 300 black and Latino sedentary adults over a 1 year period. Overall, participants will be tracked from 1 week before they start self-initiated lifestyle moderate-intensity PA through 52 weeks after the start date. They will be tracked for 3 continuous weeks using EMA and after that time, participants will be assessed via EMA and accelerometer for 1-week intervals at weeks 8, 26, 39, and 52. In addition, participants will undergo objective measurements of fitness, body fat, blood pressure, and body mass index, and will receive previously-tested intervention tools (e.g., pedometers, PA prescription plan, and newsletters). Participants' address and select socioeconomic and physical environmental characteristics (e.g., parks) of each participant's neighborhood will also be mapped using geographic information systems (GIS) in order to further contextualize their PA opportunities and resources. The primary specific aims are to: 1) examine the influence of social contextual, and perceived and objective environmental factors on the adoption and maintenance of self-initiated lifestyle moderate-intensity PA among sedentary minority adults; 2) examine whether psychosocial, social contextual, and perceived and objective environmental factors change during the course of self-initiating lifestyle moderate-intensity PA; and 3) examine the extent to which selected psychosocial factors mediate or moderate the relationships between individual differences in social contextual and environmental factors and PA. There have been few studies examining the mechanism underlying PA adoption and maintenance in minority populations. The proposed study will make a unique contribution, addressing key recommendations from major reports on reducing health disparities.